The microelectronics industry has been confronted with the problem of developing a wafer cleaning process that can better meet the needs associated with higher scales of microelectronic integration, namely contaminant-free, atomically smooth surfaces at a low cost of ownership.
In the process of fabricating a microelectronic device, such as a transistor or memory circuit, a silicon wafer undergoes many processing steps. Often, in the course of performing their function, these steps leave behind residual materials--i.e. particles, organic compounds, metals and/or metallic compounds, and low-quality SiO.sub.2 layers--that must be removed before subsequent steps can be performed.
Presently, the preferred method of removing contaminants from a wafer surface is based on the original RCA clean, Kern, W., and D. Puotinen, "Cleaning Solutions Based on Hydrogen Peroxide for use in Silicon Semiconductor technology," RCA Review, vol. 31, p. 187 (1970), Kern, W., "The evolution of silicon wafer cleaning technology," J. Electrochem. Soc., vol. 137, no. 6, p. 1887, June (1990), or a modification thereof. The RCA clean is typically a 3-step process consisting of NH.sub.4 OH/H.sub.2 O.sub.2 /H.sub.2 O (standard clean 1 or SC1), HCl/H.sub.2 O.sub.2 /H.sub.2 O (standard clean 2 or SC2), and HF/H.sub.2 O. Originally the HF step was executed between the SC1 and SC2 steps, but it is now common for the HF step to occur after these steps. SC1 and SC2 leave behind a thin layer of chemically grown silicon dioxide, which is of a poorer electrical quality than thermally grown silicon dioxide. This chemical oxide may interfere with the fabrication or electrical performance of the device. Thus, the final step in the clean is a dip in 1-2% aqueous HF to remove the chemical oxide prior to further processing.
There are primarily four problems encountered with the RCA clean and its derivatives. First, these RCA cleans are liquid phase and thus are not compatible with cluster tool environments. A cluster-tool is a system which allows the wafers to move through the process sequence without being exposed to ambient air and is often run under vacuum conditions. Second, these RCA cleans cannot produce a purely contaminant-free, damage-free surface, Ohmi, T., M. Miyashita, M. Itano, I. lmaoka, and I. Kawanabe, IEEE Electron Dev. Lett., vol. 39, no. 3, p. 537-545 (1992); Tong, J. K., G. F. Hanger, D. S. Becker, and D. J. Syverson, "A Study of the Effectiveness of Vapor Phase Cleans for Advanced Gate Oxidations," in Electrochemical Society Proceedings Volume 95-20, p. 194 (1995). The HF/H.sub.2 O dip removes the chemical oxide formed in the SC1 and SC2 steps. However, the aqueous HF can introduce atomic metal contaminants because of the difficulty in obtaining aqueous HF at the purity needed for VLSI applications. Furthermore, the clean is conducted in ambient air which leaves a thin layer of native oxide on the surface of the freshly cleaned wafer. Third, water and chemical usage in these cleans is high, and, as a result, extreme amounts of low-level waste are produced. This adds greatly to the environmental impact of the RCA clean and its overall cost of ownership. Fourth, as features on the surface decrease in size, removing the contaminated cleaning agents by rinsing becomes an increasingly more severe issue: capillary forces may prevent complete wetting of the surface with the cleaning agent, and the low mass diffusivities in a liquid phase may make the time scale necessary for cleaning impractical.
A vapor phase oxide etch can mitigate some of the shortcomings of a liquid phase etch. Primarily, vapor phase processes are compatible with the cluster tool, eliminating the need to remove the wafer from the vacuum environment and thereby reducing contamination from air-borne contaminants, water vapor, and manual wafer handling. Compared to the traditional liquid phase cleans, gas phase cleans consume less water, thus generating less waste; have no capillary forces to prevent wetting and have much higher mass diffusivities than liquids, facilitating the removal of complexed species. Gas phase etches also have the potential to do less damage to the surface in the process of cleaning it.
However, this potential is not shared by all gas phase processes. For example, plasma processes and processes using ultraviolet (UV) light each use energetic chemistries which lead to surface damage. A reduction in surface damage can only be realized by selecting reactants that chemically target only the oxide or contaminant, leaving the silicon undamaged.
Past attempts to realize a gas phase oxide etch have focused on adapting the original RCA wet-clean formulation to a dry environment. Anhydrous HF (AHF) combined with water vapor can successfully remove SiO.sub.2 in a fashion compatible with a vacuum environment, Kuiper, A. E. T., and E. G. C. Lathouwers, "Room-Temperature HF Vapor-Phase Cleaning for Low-Pressure Chemical Vapor Deposition of epitaxial Si and SiGe Layers," J. Electrochem. Soc., vol. 139, no. 9, p 2594 (1992); Muscat, A. J., A. S. Lawing, and H. H. Sawin, "Characterization of Silicon Oxide Etching in Gas Phase HF/Vapor Mixtures," in Electrochemical Society Proceedings Volume 95-20, p. 371 (1995). However, it has no capacity for removing metallic species. Thus the AHF/H.sub.2 O treatment is followed by either an in-situ DI water and dry step Ueda, et. al., "Vapor Phase Wafer Cleaning system; Edge 2000," The Sumitomo Search, no. 47, p. 134 (1991); Mehta, J. R., T. Rogers, and Satoshi Kikuchi, "Selective Etching for Making Cylindrical Capacitors Using Anhydrous HF Vapor Phase Chemistry," in Electrochemical Society Proceedings Volume 95-20, p. 194 (1995); Bhat, U.S. Pat. No. 5,589,422 (1996); Bohannon, B., B. Witowski, J. Barnett, and D. Syverson, "Vapor Phase Cleaning of Polysilicon Emitter and Titanium Silicide Structures for 0.35 Micron Technologies," in Proceedings Of The Third International Symposium On Cleaning Technology In Semiconductor Device Manufacturing (Proc. Electrochem. Soc., Pennington, N.J.), p. 362 (1994) or a treatment with UV light and either Cl.sub.2, Daffron, C., K. Torek, J. Ruzyllo, and E. Kamieniecki, "Removal of Al from Silicon Surfaces Using UV/Cl2, " in Proceedings Of The Third International Symposium On Cleaning Technology In Semiconductor Device Manufacturing (Electrochem. Soc., Pennington, N.J.), p. 281 (1994) or SiCl.sub.4 Sugino, Rinshi, et. al., "Removal of Fe and Al on a Silicon Surface Using UV-Excited Dry Cleaning," IEICE Trans. Electron., vol. E75-C, no. 7, p. 829 (1992); Sugino, U.S. Pat. No. 5,178,721 (1993); Sugino and Ito, U.S. Pat. No. 5,221,423 (1993)) to remove the metal species.
The DI water rinse/dry is not fully vacuum compatible because of the introduction of appreciable amounts of liquid water into the chamber.
The UV/Cl.sub.2 treatment creates active chlorine species which react with the silicon surface leading to surface roughness at an atomic scale.
The UV/SiCl.sub.4 treatment has a similar effect where the SiCl.sub.4 is activated in a process which is poorly understood, leading to similar damage of the silicon surface. This damage degrades the electrical performance of the device or imposes limitations on the scale of the device.
One permutation of this approach to a fully integrated oxide etch is the substitution of an alcohol for the water. To date, only the use of short-chain aliphatic alcohols has been reported in the literature: methanol, Torek, K. J. Ruzyllo, and E. Kamieniecki, "Silicon Surfaces Exposed to Anhydrous HF/CH3OH Etching," in Proceedings Of The Third International Symposium On Cleaning Technology In Semiconductor Device Manufacturing (Electrochem. Soc., Pennington, N.J.), p. 384 (1994); Torek, K., A. Mieckowski, and J. Ruzyllo, "Evolution of Si Surface after Anhydrous HF:CH3OH Etching," in Electrochemical Society Proceedings Volume 95-20, p. 208 (1995); Izumi, A, et. al., "A New Cleaning Method by Using Anhydrous HF/CH3OH Vapor System," in Proceedings Of The Second International Symposium On Cleaning Technology In Semiconductor Device Manufacturing (Electrochem. Soc., Pennington, N.J.), p. 260 (1992); Ma, Y, and M. Green, "Integrated Pre-Gate Dielectric Cleaning and Surface Preparation," in Advances in Rapid Thermal and Integrated Processing, F. Roozeboom (ed.), p. 217 (1996), ethanol, Garrido, B., J. Montserrat, and J. R. Morante, "The Role of Chemical Species in the Passivation of &lt;100&gt; Silicon Surface by HF in Water-Ethanol Solutions," J. Electrochem. Soc., vol. 143, no. 12, p. 4059 (1996), and propanol, de Larios, J. M., and J. O. Borland, "Selective etching of Native Oxide Using Vapor HF Processing," in Proceedings Of The Third International Symposium On Cleaning Technology In Semiconductor Device Manufacturing (Electrochem. Soc., Pennington, N.J.), p.347 (1994); Butterbaugh, J. W. C. F. Hiatt, and D. C. Gray, "Gas-Phase Etching of Silicon Oxide with Anhydrous HF and Isopropanol," in Proceedings Of The Third International Symposium On Cleaning Technology In Semiconductor Device Manufacturing (Proc. Electrochem. Soc., Pennington, N.J.), p. 374 (1994).
Presumably, the alcohol behaves in a fashion mechanistically similar to H.sub.2 O with respect to the functionality of the --OH group, Muscat, A. J., A. S. Lawing, and H. H. Sawin, "Characterization of Silicon Oxide Etching in Gas Phase HF/Vapor Mixtures," in Electrochemical Society Proceedings Volume 95-20, p. 371 (1995); Izumi (1992); Ma (1996)) to polarize the Si--O bonds. The advantage is that the alcohols have a higher vapor pressure than water, thus aiding the removal of residual alcohol from the wafer and chamber prior to the next step of processing. The disadvantage is that the alcohol also has no more capacity to remove metal species than does H.sub.2 O, and thus the wafer must be further cleaned either by UV/Cl.sub.2 or a liquid phase DI water rinse/dry.
U.S. Pat. No. 5,094,701 discloses a metal-containing contaminant cleaning process for silicon wafers using .beta.-diketones or .beta.-diketoimines in an oxidizing atmosphere of oxygen, air, HCl, Br.sub.2, Cl.sub.2, and HF in a carrier gas of argon, nitrogen, helium and perfluorocarbons. The cleaning process is conducted at 200.degree. C. to 300.degree. C. and the sole example is run at 205.degree. C. Silicon oxide removal is not disclosed or contemplated.
U.S. Pat. No. 5,221,366 teaches a process for etching a metal substrate of metal surface materials using .beta.-diketones or .beta.-diketoimines in an oxidizing atmosphere of oxygen, air, HCl, Br.sub.2, Cl.sub.2, and HF in a carrier gas of argon, nitrogen, helium and perfluorocarbons. The cleaning process is conducted at 200.degree. C. to 300.degree. C. and the sole example is run at 240.degree. C. Silicon oxide removal is not disclosed or contemplated.
E. A. Robertson III, S. E. Beck, M. A. George, D. A. Miniot, D. A. Bohling, "Chemical Vapor Cleaning of Fe and Cu from Silicon Wafer Surfaces, Paper No. MS-MoP7", AVS Nat'l Symposium, Oct. 14, 1996, was a presentation on etching of metals, in which HF and 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (Hhfac) were used at 300.degree. C. and 7.6 Torr on a metal contaminated silicon oxide surface. These conditions would be inappropriate for formation of active silicon oxide etch species from HF and Hfac. Similar results were reported in S. E. Beck, M. A. George, D. A. Bohling, E. A. Robertson III, D. A. Miniot, J. L. Waskiewicz, A. J. Kobar, "The Use of .beta.-Diketones to Remove Transition Metals", Tenth Annual Dielectrics and CVD Metallization Symposium Schumacher, Mar. 2/3, 1998, San Diego, Calif.
U.S. Pat. No. 5,626,775 describes a plasma-based etch process of silicon oxide and other substrates from silicon wafers and other substrates using trifluoroacetic acid, trifluoroacetic anhydride, trifluoroacetic acid amide, and trifluoromethyl ester of trifluoroacetic acid. The plasma conditions include temperatures of 25.degree. C. to 500.degree. C. and an oxygen atmosphere. The plasma conditions would destruct the trifluoroacetic reagents during the cleaning process as alluded to at the base of column 4 of the patent. Metal contaminant removal is not disclosed or contemplated.
International Patent Appln. No. WO97/257 discloses a process for a UV activated process for removing metals from a substrate surface with .beta.-diketones and .beta.-diketoimines.
The semiconductor industry has been confronted with the problem of simultaneous removal of silicon oxides and metal contaminants from silicon wafers and the like in a process which has less environmental impact, more effectively removes the desired materials, requires less energy input such as from UV or plasma generators and results in a cleaner resulting substrate product. The present invention overcomes the problems of the prior art and achieves the goals of the semiconductor industry by providing an improved thermal, vapor phase process to simultaneously remove silicon oxides and metal contaminants from substrates, such as silicon wafer surfaces, as will be set forth in greater detail below.